1. Field of the Invention
The present invention relates to a wet friction material used in a frictional engagement device such as a clutch or a brake, used in oil in an automatic transmission or the like, for a vehicle such as a car.
2. Background Art
An automatic transmission in a vehicle such as a car generally includes a multiple disk clutch in which a plurality of friction plates each having a wet friction material bonded to a surface of a metal substrate (core plate) and a plurality of separator plates as a subject material of friction each constituted by a single plate or the like such as a metal plate are disposed alternately. In an ATF (automatic transmission fluid) used as lubricating oil, these plates are frictionally connected/disconnected to/from one another so that driving force is transmitted/released.
A paper type wet friction material called xe2x80x9cpaper friction materialxe2x80x9d is generally used as the wet friction material used in the frictional engagement device used in such oil. The wet friction material is generally produced by the steps of: making wet paper from a fiber base material of natural pulp fiber, organic synthetic fiber, inorganic fiber, etc. and a filler and friction adjustor such as diatomaceous earth, cashew gum, etc.; impregnating the wet paper with a resin binder of a heat-curable resin; and thermally hardening the wet paper.
In the aforementioned paper type wet friction material, the heat-curable resin used as a resin binder with which the paper body is impregnated has not only a function of binding and holding the fiber base material and the filler, etc. but also a strong influence on friction property, abrasion resistance, etc. of the wet friction material. Hence, an unmodified phenol resin was heretofore mainly used as the resin binder because the unmodified phenol resin was excellent in heat resistance, high in physical strength and relatively good in abrasion resistance.
Incidentally, reduction in weight of various kinds of parts and increase in efficiency thereof have been promoted in pursuit of energy saving and weight reduction in the recent car industry. On the other hand, a car engine has a tendency to be increased in its rotational speed and output. Also in the automatic transmission, the wet friction material has been required to be enhanced in friction coefficient and to be more greatly improved in heat resistance and durability to be adapted for reduction in size and weight of the frictional engagement device and increase in rotational speed and output of the car engine. Incidentally, the background-art wet friction material using an unmodified phenol resin as a binder was high in crosslink density and hard. Hence, the local butting of a frictional surface of the wet friction material made the initial fluctuation of the friction coefficient large so that the friction coefficient was small initially but increased because of the conformability of the frictional surface as the frictional surface was used. Moreover, under the condition of high temperature and high load, burnt portions, called heat spots, due to the high temperature of the frictional surface were generated in the object of friction (separator plate). Hence, there was a problem that long-term stability in friction property was insufficient. The problem needed to be solved.
To solve the aforementioned problems, there have been investigated various kinds of resins such as a silicone resin, etc. other than a modified phenol resin and a phenol resin. Of the various kinds of resins, the silicone resin heretofore investigated as a binder for the friction material mainly uses organochlorosilanes as its raw material. In addition to excellent heat resistance and durability due to main skeleton siloxane bond, the silicone resin is considered an excellent material, in place of the phenol resin, as a binder for the paper type wet friction material because characteristic in a wide range of from flexibility to rigidity can be exhibited by adjustment of crosslink density due to the design of mix of organochlorosilanes as a raw material. The silicone resin is, however, inferior to the phenol resin in permeability and wettability to the paper base material having fine porosity because the silicone resin is generally composed of an organic material and an inorganic material. Accordingly, the paper type wet friction material using the aforementioned silicone resin as a binder lacks physical strength. No satisfactory wet friction material has ever been obtained and has ever been put into practical use.
Therefore, an object of the present invention is to provide a wet friction material excellent in heat resistance and durability, high in friction coefficient, small in initial fluctuation of the friction coefficient, excellent in heat spot resistance and superior in physical strength to a wet friction material using a phenol resin as a binder.
To solve the aforementioned problem, the inventors of the present invention have eagerly investigated the hydrolyzed solution of a silane coupling agent with attention paid to the fact that the silane coupling agent is widely used for improving performance of a composite material containing an organic polymer and an inorganic and/or metal material. The silane coupling agent is represented by the general formula Yxe2x80x94SiX3 in which Y shows a reactive organic functional group represented by an amino group, an epoxy group, a vinyl group, a methacryl group or a mercapto group, and X shows a hydrolyzable group represented by an alkoxy group. The mechanism of action of the silane coupling agent is as follows. The hydrolyzable group X, for example, an alkoxy group reacts with water to produce a silanol group to be bonded to a hydroxy group in a surface of the inorganic material. On the other hand, the reactive organic functional group Y, for example, an amino group reacts with a reactive group contained in the organic polymer, so that the two groups are chemically (covalently) bonded to each other. That is, the silane coupling agent acts as an intermediary between the inorganic material and the organic material to thereby effectuate enhancement of physical strength, enhancement of the affinity of the inorganic material for the organic resin, suppression of lowering of physical strength under the condition of high temperature and high humidity, and so on. For application of the silane coupling agent having the aforementioned characteristic to a paper type wet friction material, the silane coupling agent must be particularly excellent in permeability and wettability to a paper base material having fine porosity because the wet friction material needs to be impregnated with a binder uniformly as a whole. The inventors have eagerly investigated the hydrolyzed solution of such a silane coupling agent from this point of view. As a result, they have found that a silane coupling agent represented by the general formula Yxe2x80x94SiX3 is particularly excellent in permeability and wettability to the paper base material when Y and X are an amino group and an alkoxy group, respectively. Thus, they have accomplished the present invention.
That is, according to the present invention, there provided a wet friction material comprising a fiber base material, a filler, a friction adjustor, and a binder; the binder is constituted by a hardened substance of a hydrolyzed solution of a silane coupling agent represented by the formula (2)
(R1)(R2)nSi(OR3)3xe2x88x92nxe2x80x83xe2x80x83(2)
in which R1 represents an alkylamino group containing primary amine at a terminal, each of R2 and R3 represents an alkyl group containing 1 to 3 carbon atoms, and n represents an integer of 0 or 1.
Here, the silane coupling agent is constituted by a silane coupling agent containing three hydrolyzable groups in one molecule or by a mixture of a silane coupling agent containing three hydrolyzable groups in one molecule and a silane coupling agent containing two hydrolyzable groups in one molecule.
Preferably, when the silane coupling agent is constituted by a mixture of a silane coupling agent (n=0) containing three hydrolyzable groups in one molecule and a silane coupling agent (n=1) containing two hydrolyzable groups in one molecule, the hydrolyzed solution of the silane coupling agent is prepared so that the molar ratio of the silane coupling agent containing two hydrolyzable groups in one molecule to the silane coupling agent containing three hydrolyzable groups in one molecule is not higher than 10.
Further preferably, the hydrolyzed solution of the silane coupling agent contains water with an amount not smaller than the amount permitting hydrolyzing for half the number of hydrolyzable groups contained in the silane coupling agent but not larger than twice as much as the amount permitting hydrolyzing for all the number of hydrolyzable groups contained in the saline coupling agent.